Yes, I understand that these motorhomes getting this alleged mpg are diesel. I know you hate gas engines, and that's ok. I like diesels. I own a diesel pickup. It weighs about 7000 lbs., and at 60 mpg on flat terrain it gets 20 mpg all day. It's older but is in excellent mechanical shape and has no emission controls. My buddy has a Mercedes Sprinter diesel class B motorhome, and he doesn't get anywhere near 26 mpg. Maybe Australian math is different?
Several things you did not get, the diesel regulations outside NA are very different to the US, so diesels are not being " strangled" by regulations. It is like comparing Apples to Iranges. You could not get 26mpg US using a US Diesel under your regulations, in that Australian Motorhome. People do not " hate" Gas engines here, bulk of vehicles running are Petrol powered, but a Petrol powered Motorhome? No way.
20mpg for a Diesel Pickup is a bit subpar, think we are on a different page